Tunable ultra high frequency apparatus



Feb. 4, 1947..

L. B. MALLING 'rmiiinm uLm-nmmmqmcr Arrm'ws Filed Se t. 25. 1943 FIG.2

INVENTOR LEONARD R. MALLING A; izR

" Patented Feb. 1947 TUNABLE ULTRA HIGH FREQUENCY APPARATUS Leonard R.Malling, Douglaston, N. Y., assignor, by mesne assignments, to HazeltineResearch, 7 Inc., Chicago, Ill., a corporation of Illinois ApplicationSeptember 25, 1943, Serial No. 503,766

This invention relates to a tunable ultra-highfrequency apparatusincluding a plurality of effectively resonant transmission lines tunableover given ranges of operating frequencies in manner which maintains apredetermined constant relation between their individual operatingfrequencies. Although the invention is subject to a wide range ofapplications, it is especially suited for use in an ultra-high-frequencyoscillation generator of the directly heated cathode type and will bedescribed in such connection.

In oscillation generators of the directly heated cathode type, theheater circuit forms part of both the anode-cathode and grid-cathodecircuits. For low-frequency operation where the heater circuit isprimarily resistive, this arrangement is satisfactory. However, in theultra-high-frequency range the distributed inductance and capacitance ofthe leads which connect a suitable heater voltage to the cathode causethe heater circuit to be reactive at all except its resonant frequency.For any operating condition in which the heater circuit is primarilyreactive, an undesirable phase shift is introduced into the feed-backcircuit of the oscillator, impairing its operating efllciency and insevere cases blocking completely the generation of oscillations. Forthis reason oscillation generators of the type under consideration havea limited operating frequency range unless some provision is made forcompensating or avoidin the reactive effects of the heater circuit.

In prior art arrangements a short-circuited ef fectively resonanttransmission line is frequently included in the heater circuit to avoidits aforementioned undesirable reactive effects. The line usually istuned to have an effective electrical length equal to a half wave lengthcorresponding to the oscillator operating frequency and, therefore, isprimarily resistive, exhibiting the properties of a series-resonantcircuit. Where the fre truancy-determining circuit of the generatorcomprises a second short-circuited effectively resonant transmissionline, which is the usual case, the second line also generally hasdistributed circuit constants, but has an effective electrical lengthequal to a quarter wave length corresponding to the operating frequencyof the oscillator. Since the lines have different effective electricallengths, the movements of their respective short-circuiting elements toeffect identical changes in the operating frequencies of the lines aredissimilar. This is an undesirable limitation of such prior artarrangements in that it precludes the use of a simple unicontrolmechanism for tuning the oscillator over its range of operatingfrequencies.

13 Claims. (Cl. 250-36) It is an object of the invention, therefore, toprovide an improved tunable ultra-high-frequency apparatus including aplurality of eifectively resonant transmission lines and which avoidsthe aforementioned limitations of prior art arrangements.

It is another object of the invention to provide an improved tunableultra-high-frequency apparatus comprising a plurality of tunableeffectively resonant transmission lines of different effectiveelectrical lengths and having a simplified unicontrol' mechanism fortuning the apparatus over a given range of operating frequencies.

In accordance with the invention, a tunable ultra-hlgh-frequencyapparatus comprisesa first effectively resonant transmission line havinan effective electrical length equal to an integral multiple of aquarter wave length corresponding to a predetermined operatingfrequency. This first line includes adjustable tuning means for varyingits effective electrical length to tune the line over a given range ofoperating frequencies. The apparatus also comprises a, secondefiectively resonant transmission line uniformly loaded throughout atleast a portion of its length and having an effective electrical lengthequal to an integral multiple of a; quarter wave length corresponding toa predetermined operating frequency. The second line also includesadjustable tuning means in the uniformly loaded portion thereof forvarying its effectivev electrical length to tune the second line over agiven range of operating frequencies and unicontrol means are providedfor effecting substantially identical physical adjustments of the tuningmeans of the first and second lines. Additionally, the loading of thesecond line is such that a predetermined constant relation is maintainedbetween the operatin fre-- quencies of the first and second lines asthese lines are tuned over their ranges of operating frequencies.

As employed throughout this specification and in the appended claims,the term effectively resonant transmission 'line refers to atransmission line and its terminating impedance which together exhibitcertain desirable characteristics at or near its resonant, frequency,which frequency is determined by the distributed parameters of the lineand its terminating impedance.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawing, and itsscope will be pointed out in the appended claims.

In the drawing, Fig. 1 is a perspective view, partially schematic, of atunable ultra-high-frequency apparatus embodying the present inventionin a preferred form; Fig. 2 is a schematic circuit diagram which is theelectrical equivalent of the circuit of Fig. l; and Fig. 3 is afragmentary perspective view of a portion of the apparatus of Fig. 1 inmodified form.

Referring now more particularly to Fig. 1, there is represented atunable ultra-high-frequency apparatus, specifically an oscillationgenerator of the directly heated cathode type, operable over a givenrange of operating frequencies and embodying the present invention. Theapparatus comprises a vacuum-tube oscillator l0, representedschematically, having an anode, a control electrode, and a directlyheated cathode, and a frequency-determining circuit provided by a firsteffectively resonant transmission line H directly connected to the anodeand control electrode of the vacuum tube so as to beterminated by theirinterelectrode capacitance Cg-p. Transmission line H has an effectiveelectrical length equal to an odd integral multiple of a quarter wavelength corresponding to a predetermined operating frequency, the integerfor the illustrated embodiment preferably being equal to one. Thus, theeffective electrical length of the line ll equals a quarter wave lengthcorresponding to the operating frequency of the oscillation generator.This transmission line includes a pair of horizontally disposed parallelconductors l2 and I3 of uniformly distributed inductance and capacitanceand an adjustable tuning means, comprising a short-circuiting member,for varying their effective electrical length to tune the line over agiven range of operating frequencies determined by the desired operatingfrequency range ofthe apparatus. Conductors I2 and I3 are supported inbut insulated from extensions of the end walls of a metal container 30,the line insulators being omitted in order to simplify the drawing. Theshort-circuiting member for varying their efiective electrical lengthconsists of conductive elements [4 and I5 secured to the horizontalportion of a supporting yoke 16 of suitable insulating material, such asa thermoplastic resin. Elements l4 and [5 are individually apertured toreceive and be freely movable along conductors I2 and I3. A mica stripI1 is positioned intermediate the adjacent ends of elements i4 and I5effectively to constitute the described short-circuiting member acondenser which completes the circuit of line I I for alternatingcurrents. A condenser I8 is bridged across those terminals of conductorsl2 and I3 which are remote to tube Ill for the purpose of detuning thatportion of line H located between suchremote terminals andshort-circuiting member l4l5ll.

The cathode of vacuum tube In is heated by means of a suitable battery,indicated A, through a heater circuit that includes a second effectivelyresonant transmission line 20 uniformly loaded throughout at least aportion of its length and havingan effective electrical length equal toan even integral multiple of a quarter wave length corresponding to apredetermined operating frequency. For the embodiment under considera-'tion, transmission line 20 is loaded throughout a major portion of itslength by dielectric means, in a, manner described more particularlyhereinafter, and has an effective electrical length equal to a half wavelength corresponding to the operating frequency of the oscillator. Thistransmission line similarly comprises a.- pair of parallel conductors 2|and '22, a short-circuiting member including conductive elements 23 and24, an interposed strip of mica 25, and a detuning condenser 28connected across the ends of conductors 2| and 22 which are remote totube l0. These elements have the same construction as correspondingelements of transmission line H. Also, line 20 is supported in, butinsulated from, the end' walls of container 30 through insulatingbushings (not shown). Short-circuiting member 23-44-45 is grounded tothe metal container 30 through a resilient U-shaped brush 29 which isadapted to engage a side wall of the container and be freely slidabletherealong.

In general, the. terminating impedance of line 20 provided by theimpedance of the cathode of tube In is materially different from theterminating impedance of line H. As a consequence, it is necessary todisplace the short-circuiting members of the lines, as indicated by thedimension line d in the drawing, to have the lines tuned to the sameoperating frequency. For this purpose short-circuiting member 2324-25 ofline 20 is adjustably secured to supporting yoke (6 through an insulatedU-shaped member 26 having elongated slots 21.

Uniform loading of line 20 throughout a major portion of its length isaccomplished by immersing that portion thereof in a homogeneousdielectric material. To this end, that portion of line 2|] is supportedwithin the container 30, the container being filled with a liquiddielectric, such as oil, to such a depth, indicated by broken line 3| inthe drawing, that the portion of line 20 contained therein is completelyimmersed.

The oil utilized is chosen to have such a dielectric constant that equalincremental displacements of shorting bar l4l 5I 1 and 23-2425 alonglines II and 20, respectively, cause 'such variations in the operatingfrequencies of the lines that a predetermined constant relation ismaintained between the operating frequencies of the lines as they aretuned over their individual operating frequency ranges. For thedescribed embodiment of the invention, the dielectric is chosen of suchvalue that identical changes are effected in the operating frequenciesof lines H and 20 for equal physical adjustments of their individualshort-circuiting members.

The apparatus further comprises unicontrol means for effectingsubstantially identical physical adjustments of the tuning mean of linesII and 20. Such unicontrol means consists of a threaded rod 33 engaginga threaded aperture in yoke I6 and suitably supported in bearings 34'and 35 provided in end walls of container 30. The rod 33 is terminatedin a knurled knob 36 which facilitates precise adjustment of yoke l6 andexact tuning of the apparatus. A grid-leak 0 resistor 32 is coupled tothe control electrode of vacuum tube l0 through conductor l2 and a.source of space current indicated +B is coupled to the anode thereofthrough conductor [3.

In considering the operation of the described arrangement, it will beseen that, for a given operating condition, transmission line 20 has aneffective electrical length equal to a half wave length corresponding tothe instant operating frequency of the oscillation generator. It is wellknown in transmission network theory that a short-circuited effectivelyresonant transmission line having such an effective electrical lengthexhibits impedance characteristics analogous to those of aseries-resonant circuit, that is, it has zero series impedance at theresonant frequency.

I Therefore, subject to the usual assumptions that the resistance of thecathode element and transmission line 20 is negligible, the cathode iseffectively at ground potential. Accordingly, the oscillation generatoris of the type represented schematically in Fig. 2, wherein thedistributed inductance of conductors l2 and I 3 of transmission line His represented by the lumped inunicontrol means 33 toeffect suchmovement of short-circuiting member l4-|5-l1 produces corresponding andequal displacements of shortcircuiting member 23-24-25. Thus, it will beseen that, by virtue of its selected uniform loading which is difierentfrom that of line H, line 20 is tuned to substantially the samefrequency as line I I and, therefore, to the operating frequency of theoscillation generator as the generator is tuned throughout its range ofoperating frequencies. In other words, the circuit diagram of Fig. 2represents the equivalent circuit of the generator for any tuningcondition. Hence, the cathode of tube III is effectively maintained atground potential for all operating conditions, thereby to ensure maximumoscillator efficiency.

While applicant does not intend to limit the invention to any specificcircuit constants, the following are given for the purposes ofillustration:

Tube Ill Type 368-A Conductors l2, I3, 2!, and

22 rod 6" long separated /4 Condenser C17 and condenser C24 100micro-microfarads each Condenser C18 and condenser C23 250micro-microfarads each Distance (2 1 centimeter Loading material and itsdielectric constant Castor oil-4.6 Frequency range 500-1500 megacyclesReference is now made to Fig. 3 which illustrates a modified form of theinvention. The

'modification includes a pair of effectively resonant transmission linessimilar to those of Fig. 1, corresponding components thereof havingidentical reference numerals primed. For the purpose of simplicity, onlya portion of the lines is represented, but it will be understood thattheir construction and arrangement may be the same as those of Fig. 1.Loading of line 20 is achieved through reactance means comprising aplurality of substantially identical capacitors 40 uniformly spacedalong the line. Loading capacitors 40 are selected of such value as tomaintain a predetermined constant relation between the operatingfrequencies of lines H and 20' as the lines are tuned over their rangesof operating frequencies in a manner particularly described withreference to the loading of line 20 in the Fig. 1 arrangement. Thetuning arrangement of Fig. 3 may be substituted for that of Fig. 1 inwhich case condensers 40 are selected to maintain lines II and 20' atthe same operating frequencies as the oscillation generator is tunedover its operating frequency range.

In the preferred embodiment of the invention, short-circuiting membersM--l5-|l and 23- 24-25 are displaced with reference to one another tocompensate for the dissimilar terminating impedances of lines II and 20.However, it will be understood that a compensating impedance may beshunted across the open end of line 20 to cause the line terminations tobe substantially identical. In that case, the shorting bars l4-l5-Il and2324-25 may be in. vertical alignment, simplifying the construction ofsupporting members It and 26.

It will be understood that the transmission lines II and 20 may take theform of coaxial lines. In this case, uniform loading of one of the linesmay be accomplished by filling a portion thereof with a 'liquiddielectric material, such as oil.

While in the described embodiment of the invention, it is desirable soto load one of the transmission lines that the first and second lineshave the same operating frequencies, it will be understood that thisfrequency relationship is an illustrative but not exclusive one. In someapplications, for example in the case of an. oscillator-modulatoroperable over a predetermined range of frequencies in theultra-high-frequency spectrum, the invention may be utilized to maintaina predetermined constant frequency difference between the operatingfrequencies of. two effectively resonant transmission lines.

Also, it is well known that a short-circuited effectively resonanttransmission line having an effective electrical length equal to aquarter wave length corresponding to a predetermined operating frequencyexhibits the same impedance characteristics as an open-circuitedefiectively resonant transmission line having an effective electricallength equal to a half wave length corresponding to such operatingfrequency. Likewise, a short-circuited effectively resonant transmissionline having an effective electrical length equal to a half wave lengthof a predetermined operating frequency presents the same impedancecharacteristics as an open-circuited effectively resonant transmissionline having an effective electrical, length equal to a quarter wavelength of such operating frequency. Such equivalent lines may beutilized in a tunable ultrahigh-frequency apparatus in accordance withthe invention so long as one line is appropriately loaded in the mannerhereinbefore described.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modificationsas fallwithin the true spirit and scope of the invention.

What is claimed is:

1. A tunable ultra-high-frequency apparatus comprising, a firsteffectively resonant transmis sion line having an effective electricallength equal to an integral multiple of a quarter wave lengthcorresponding to a predetermined operating frequency, said first lineincluding adjustable tuning means for varying its effective electricallength to tune said line over a given range of operating frequencies, asecond effectively resonant transmission line uniformly loadedthroughout at least a portion of its length and having an effectiveelectrical length equal to an integral multiple of a quarter wave lengthcorresponding to a predetermined operating frequency, said second lineincluding adjustable tuning means in said uniformly loaded portionthereof for varying its effective electrical length to tune said secondline over a given range of operating frequencies, and unicontrol meansfor effecting substantially identical physical adjustments of saidtuning means of said first and second lines, said loading of said secondline being such that a predetermined constant relation is maintainedbetween the operating frequencies of said first and Second'lines 'assaid lines are tuned over said ranges of operating frequencies.

2. A tunable ultra-high-frequency apparatus comprising, a firstshort-circuited effectively resonant transmission line having aneffective electrical length equal to anintegral multiple of a quarterwave length corresponding to a predetermined operating frequency, saidfirst line including adjustable tuning means for varying its effectiveelectrical length to tune said line over a given range of operatingfrequencies, a second effectively resonant transmission line uniformlyloaded throughout at least a portion of its length and having aneffective electrical length equal to an integral multiple of a quarterwave length corresponding to a predetermined operating frequency, saidsecond line including adjustable tuning means in said uniformly loadedportion thereof for varying its effective electrical length to tune saidsecond line over a given range of operating frequencies, and unicontrolmeans for effecting substantially identical physical adjustments of saidtuning means of said first and second lines, said loading of said secondline being such that a predetermined constant relation is maintainedbetween the operating frequencies of said first and second lines as saidlines are tuned over said ranges of operating frequencies.

3. A tunable ultra-high-frequency apparatus comprising, a firsteffectively resonant transmission line having an effective electricallength equal to an integral multiple of a quarter wave lengthcorresponding to a predetermined operating frequency, said first lineincluding adjustable tuning means for varying its effective electricallength to tune said line over a given range of operating frequencies, asecond short-circuited effectively resonant transmission line uniformlyloaded throughout at least a portion of its length and having aneffective electrical length equal to an integral multiple of a quarterwave length corresponding to a predetermined operating frequency, saidsecond line including adjustable tuning means in said uniformly loadedportion thereof for varying its effective electrical length to tune saidsecond line over a given range of operating frequencies, and unicontrolmeans for effecting substantially identical physical adjustmerits ofsaid tuning means of said first and second lines, said loading of saidsecond line being such that a predetermined constant relationismaintained between the operating frequencies of said first and secondlines as said lines are tuned over said ranges of operating frequencies.

4. A tunable ultra-high-frequency apparatus comprising, a firsteffectively resonant transmission line including a pair ofparallel'conductors and having an effective electrical length equalto anintegral multiple of a quarter wave length corresponding to apredetermined operating frequency, said first line includingshort-circuiting means movable along said conductors for varying theeffective electrical length of said line to tune said line over a givenrange of operating frequencies, a second effectively resonanttransmission line including a pair of parallel conductors uniformlyloaded throughout at leasta portion of their length and having aneffective electrical length equal to an integral multiple of a quarterwave length corresponding to a predetermined operating frequency, saidsecond line including short-circuiting means movable along said loadedportion of said conductors for varying theefiecsion line having aneffective electrical length equal to an integral multiple of a quarterwave length corresponding to a predetermined operatingfrequency, saidfirst line including adjustable tuning means for varying its effectiveelectrical length to tune said line over a given range of operatingfrequencies, a second effectively resonant transmission line uniformlyloaded throughout at least a portion of it length and having aneffective electrical length equal to an integral multiple of a quarterwave length different from that of the first effectively resonanttransmission line corresponding to said predetermined operatingfrequency, said second line including adjustable tuning means forvarying its effectiveelectrical length to tune said second line oversaid range of operating frequencies, and unicontrol means for effectingsubstantially identical physical adjustments of said tuning means 'ofsaid first and second lines, said loading'of said second linebeing suchthat the operating frequencies of said first and second lines aremaintained substantially equal as said lines are tuned over said rangeof operating frequencies.

6. A tunable ultra-high-frequency apparatus comprising, a firsteffectively resonant transmission line having an effective electricallength equal to a quarter wave length corresponding to a predeterminedoperating frequency, said first line including adjustable tuning mean-sfor varying its effective electrical length to tune said line over agiven range of operating frequencies, a second effectively resonanttransmission line uniformly loaded throughout at least a portion of itslength and having an effective electrical length equal to a half Wavelength corresponding to a predetermined operating frequency said secondline including adjustable tuning means in said uniformly loaded portionthereof for varying its effective electrical length to tune said secondline over a given range of operating frequencies, and

unicontrol means for effecting substantially identical physicaladjustments of said tuning means of said first and second lines, saidloading of said second line being such that a predetermined constantrelation is maintained between the operating frequencies of said firstand second lines as said lines are tuned over said ranges of operatingfrequencies. a

7. A tunable ultra-high-frequency apparatus comprising, a firsteffectively resonant, transmission line having an effective electricallength equal to an odd integral multiple of a quarter wave lengthcorresponding to a predetermined operating frequency, said first lineincluding adjustable tuning means for varying its effective electricallength to tune said line over a given range of operating frequencies, asecond effectively resonant transmission line uniformly loadedthroughout at least a portion of its length and having an effectiveelectrical length equal to an even integral multiple of a quarter wavelength corresponding to a predetermined operating frequency, said secondline including adjustable tuning means in said uniformly loaded portionthereof for varying its effective electrical length to tune said secondline over a given range of operating frequencies, and unicontrol meansfor effecting substantially identical physical adjustments of saidtuning means of said first and second lines, said loading of said secondline bein such that a predetermined constant relation is maintainedbetween the operating frequencies of said first and second lines as saidlines are tuned over said ranges of operating frequencies.

8. A tunable ultra-high-frequency apparatus comprising, a firsteffectively resonant transmission line having an effective electricallength.

equal to an integral multiple of a quarter wave length corresponding toa predetermined operating frequency, said first line includingadjustable tuning means for varying its effective electrical length totune said line over a given range of operating frequencies, a secondeffectively resonant transmission line, dielectric means for uniformlyloading said second line throughout at least a portion of its length tocause said line to have an effective electrical length equal to anintegral multiple of a quarter wave length corresponding to apredetermined operating frequency, said second line including adjustabletuning means in said uniformly loaded portion thereof for varying itseffective electrical length to tune said second line over a given rangeof operating frequencies, and unicontrol means for effectingsubstantially, identical physical adjustments of said tuning means ofsaid first and second lines, said loading of said second line being suchthat a predetermined constant relation is maintained between theoperating frequencies of said first and second lines as said lines aretuned over said ranges of operating frequencies.

9. A tunable ultra-high-frequency apparatus comprising, a firsteffectively resonant transmission line having an effective electricallength equal to an integral multiple of a quarter wave lengthcorresponding to a predetermined operating frequency, said first lineincluding adjustable tuning means for varying its effective electricallength to tune said line over a given range of operating frequencies, asecond effectively resonant transmission line comprising a pair ofspaced parallel conductors, a homogeneous dielectric material in thespace between said conductors throughout at least a portion of theirlength for uniformly loading said second line to cause said second lineto have an effective electrical length equal to an integral multiple ,ofa

'10 eluding adjustable tuning means in said uniformly loaded portionthereof for varying its effective electrical length to tune said secondline over a given range of operating frequencies, and unicontrol meansfor effecting substantially identical physical adjustments of saidtuning means of said first and second lines, said loading of said secondline being such that a predetermined constant relation is maintainedbetween the operating frequencies of said first and second lines as saidlines are tuned over said ranges of operating frequencies.

10. A tunable ultra-high-frequency apparatus comprising, a firsteffectively resonant transmission line having an effective electricallength equal to an integral multiple of a quarter wave lengthcorresponding to a predetermined operating frequency, said first lineincluding adjustable tuning means for varying it effective electricallength to tune said line over a given range of operating frequencies, asecond effectively resonant transmission line including reactance meansfor uniformly loading said line throughout at least a portion of itslength and having an effective electrical length equal to an integralmultiple of a quarter wave length corresponding to a predeterminedoperating frequency, said second line including adjustable tuning meansin said uniform- 1y loaded portion thereof for varying its effectiveelectrical length to tune said second line over a given range ofoperating frequencies, and unicontrol means for effecting substantiallyidentical physical adjustments of said tuning means of said first andsecond lines, said loading of said second line being such that apredetermined constant relation is maintained between the operatingfrequencies of said first and second lines as said lines are tuned oversaid ranges of operating frequencies.

11. A tunable ultra-higlr-frequency apparatus comprising, a firsteffectively resonant transmission line having an effective electricallength equal to an integral multiple of a quarter wave: lengthcorresponding to a predetermined operating frequency, said first lineincluding adjustable tuning means for varying its effective electricallength to tune said line over a given range of operating frequencies, asecond effectively resonant transmission line including a plurality ofsubstantially identical and uniformly spaced capacitors for uniformlyloading said second line throughout at least a portion of its lengthand. having an effective electrical length equal to an integral multipleof a quarter wave length corresponding to a predetermined operatingfrequency, said second line including adjustable tuning means in saiduniformly loaded portion thereof for varying its effective electricallength to tune said second line over a given range of operatingfrequencies, and unicontrol means for effecting substantially identicalphysical adjustments of said tuning means of said first and secondlines, said loading of said second line being such that a predeterminedconstant relation is maintained between the operating frequencies ofsaid first and second lines as said lines are tuned over said ranges ofoperating frequencies.

l2. A tunable ultra-"lugh-frequency apparatus comprising, a vacuum tubehaving an anode, a control electrode and a directly heated cathode, afirst effectively resonant transmission line coupled to said anode andcontrol electrode and having an effective electrical length equal to anintegral multiple of a quarter wave length corresponding to apredetermined operating frequency,

a 11 said first line including adjustable tuning means for varying itseffective electrical length to tune saidline over a given range ofoperating frequencies, a heater circuit coupled to said cathodeincluding a second effectively resonant transmission'line uniformlyloaded throughout at least a portion of its length and having-aneffective electrical length equal to an integral multiple of a quarterwave length corresponding to a predeterand second lines, said loading ofsaid second line being such that a. predetermined constant rela-- tionis maintained between the operating frequencies of said first and secondlines as said lines are tuned over said ranges of operating frequencies.

13. A tunable ultra-high-frequency apparatus comprising, an oscillationgenerator including a vacuum tube having ananode, a control electrodeand a directly heated cathode, a first effectively resonant transmissionline coupled to the anode' control electrode for determining thefrequency of the generated oscillations and having an efiectiveelectrical length equal to an integral multiple of a quarter wave lengthcorresponding to a predetermined operating frequency, said first lineincluding adjustable tuning'means for varyin its effective electricallength to tune said line over a given range of operating frequencies, aheater circuit coupled to said cathode including a second effectivelyresonant transmission line uniformly loaded throughout at least aportion of its length and having an effective electrical length equal toa different integral multiple of a quarter wave length corresponding tosaid predetermined operating frequency, said second line includingadjustable tuning means in said uniformly loaded portion thereof forvarying its effective electrical length to tune said second line oversaidrange of operating frequencies, and unicontrol means for effectingsubstantially identical physical adlustments of said'tuning means ofsaid first and second lines, said loading of saidsecond line being suchthat the operating frequencies of said first and second lines aremaintained 4 substantially equal as said lines are tuned over saidranges of operating frequencies. v

' LEONARD R. MAILING.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED swims-PATENTS Number Name- Date t 2,233,763 Ayer Mar. 4, 19412,192,494 Goddard Mar. 5, 1940

